The present invention relates to a bearingless rotor system, and more particularly to a torque tube and intermediate tube blade mounting arrangement which separates the motion accommodating functions of an elastomeric snubber bearing and lead/lag bearing.
Bearingless or xe2x80x9cflexbeamxe2x80x9d rotor systems require resilient load carrying members between the flexbeam and its surrounding torque tube. The load carrying members position the flexbeam and the attached rotor blade spar for pitch change, flapping and lead/lag motion about the intersection of the pitch change and flapping axes.
The load carrying members are typically elastomeric bearings known as snubber/dampers which include vertically stacked arrangements of spherical and flat elastomeric laminates to center the torque tube about the flexbeam while allowing flapping, pitch and lead/lag motions. The flat layers are stacked to produce a cylindrical cavity to house a damper. The spherical bearing xe2x80x9csnubberxe2x80x9d accommodates pitch change and flapping rotation (as well as a small amount of lead/lag rotation) while the flat layers accommodate lead/lag linear motions and some radial(spanwise)motion.
The snubber/dampers are located between the flexbeam spar and the torque tube under a preload so that the elastomer laminates thereof remain in compression throughout the full range of articulation as the elastomeric laminates may fail under tension. The snubber/dampers are commonly mounted through a clearance opening in the torque tube and attached through an opening in the flexbeam spar. The snubber/dampers are axially preloaded by a shimming procedure. Preloading reduces the free height of the elastomeric stack while pre-stressing the torque tube. Although highly effective, difficulties arise with conventional bearingless rotor systems.
Firstly, as the blade lead/lags, the preload leads/lags which generates high bending load moments. The bending load moments may overcome the compressive preload and produce tension in the elastomeric bearing arrangement. Tension is detrimental to elastomeric laminates as tension operates to delaminate the elastomeric bearing arrangement. As lead/lag motion increases, the preload is further reduced which thereby further compounds this effect.
Secondly, the vertically stacked elastomeric bearing arrangement may in some circumstances provide poor damping. The cylindrical damper cavity cannot be easily separated into two separate chambers which would allow orifice/piston type damping as the cavity changes shape as the elastomer deforms to accommodate lead/lag motion. A typical arrangement requires two dampers (one above the flexbeam and one below) to provide symmetry and compensate for inefficiencies. Moreover, the vertically stacked structure is complex and highly stressed which results in relatively low fatigue life.
Thirdly, a pitch horn is typically attached to the torque tube to receive control inputs from a swashplate. The elastomeric bearing transfers pitch inputs and rotor flap shear in the torque tube to the flexbeam. Because the pitch horn is mounted to the torque tube, the horn radius (the distance between the pitch input and the rotor feathering axis) changes as the blade lead/lags. This may produce unstable pitch/lag coupling effects as well as undesirable control system vibration.
Lastly, consideration must also be provided for the size of the elastomeric bearing in relation to the accommodation of loads and motions involved in flight as designs which meet desired flight envelope capabilities may not be readily contained within the torque tube. Simply increasing the torque tube size would undesirably increase rotor system weight and drag.
Accordingly, it is desirable to provide a bearingless rotor system which overcomes these difficulties while improving the fatigue life of the elastomeric bearings.
The flexbeam rotor system according to the present invention provides a series of rotor blade assemblies each of which has a snubber-vibration damper system. Each blade assembly includes a flexbeam integrally connected to the rotor hub. An intermediate tube and a torque tube envelope the flexbeam in spaced relation thereto. The torque tube is connected to the flexbeam at its radially outer end and articulately connected to the intermediate tube through the snubber-vibration damper system. The torque tube is connected or integral to an aerodynamic rotor blade member.
The snubber vibration damper system includes a snubber bearing and a lead/lag bearing. The snubber bearing includes spherical bearing elements and cylindrical bearing elements. The cylindrical bearing elements are not flat, but are axisymmetric shells defined about the pitch axis to accommodate some of the pitch motion and all of the spanwise linear motion. The snubber bearing is located between the flexbeam and the intermediate tube along a pitch change axis. The torque tube is mounted for lead/lag motion relative to the intermediate tube through the lead/lag bearing. Pitch motion is directly transferred from the intermediate tube to the torque tube.
Each lead/lag bearing includes a cylindrical elastomeric bearing mounted about a mount which is rigidly attached to the intermediate tube. The lead/lag bearing is located independent of the snubber bearing. The inner diameter of the cylindrical elastomeric bearing is attached to the mount and the outer diameter is attached to the torque tube. Lead/lag motion between the torque tube and the flexbeam is thereby accommodated by the lead/lag bearing independent of the snubber bearing.
A lead/lag bearing is mounted on both the leading and trailing portion of blade assembly to provide a chord wise-distance therebetween. The offset between the leading and trailing lead/lag bearing defines an axis which is parallel to an axis defined between the center of the snubber bearing and a point which defines the center of the pitch horn. By locating the axes in parallel, a purely vertical pitch/flap force coupling results.
The leading edge damper is positioned outboard of the snubber. Damper force will produce an inplane moment about the snubber, which is reacted by spanwise forces in the cylindrical lead/lag bearings. Also, pitch inputs produce a torsional moment which are reacted by vertical forces in the lead/lag bearings. The high chordwise distance between the lead/lag bearings reduces both the spanwise forces and the vertical forces. A damper such as multi-chamber fluid damper is mounted to the leading lead/lag bearing along the leading mount. The trailing lead/lag bearing is longitudinal offset from the leading lead/lag bearing.
The pitch horn is mounted to the intermediate tube inboard of the snubber bearing. By mounting the pitch horn inboard of the snubber bearing, a desirable pitch/flap coupling (delta-3) is achieved.
Pitch/lag coupling complications are also minimized as lead/lag motion takes place between the torque tube and the intermediate tube. Unstable pitch/lag coupling effects and undesirable control system vibrations are greatly reduced. Moreover, control loads and control displacement requirements are reduced as pitch inputs are more directly communicated through the intermediate tube.
The snubber bearing includes spherical bearing elements and cylindrical bearing elements. The cylindrical bearing elements need only accommodate minimal spanwise motion of the torque tube relative the flexbeam. This motion is relatively minor in comparison to lead/lag motion. As lead/lag motion is accommodated in the independent lead/lag bearing, the snubber bearing is of a greatly reduced height in relation to conventional design. The overall size of the torque tube is accordingly reduced which advantageously reduces system weight and drag.
The intermediate tube preloads the snubber bearing independently. That is, the preload force does not lead/lag with the torque tube. It is therefore relatively less difficult to maintain the snubber bearing in compression throughout the full range of articulation. The spherical bearing elements have reduced bending load movement, less possibility of being subjected to tension and an improved fatigue life. Further, because the snubber bearing is subjected to reduced bending loads due to lead/lag, mounting is simplified.
The present invention therefore overcomes difficulties associated with conventional bearingless rotor system while improving the fatigue life of the elastomeric bearings.